This invention relates to a marine surface vessel, but more specifically, to a method and system to improve propulsive performance by reducing skin friction drag on a hull of a marine surface vessel while underway at sea.
As generally accepted in the marine science, resistance to ship propulsion due to hydrodynamic drag includes skin friction drag, wave drag, and form drag. The total hydrodynamic drag force opposing propulsion FHD=½·CRρυ2A, where CR is the coefficient of friction, ρ is density of water, υ is the relative velocity between the vessel and the water, and A is the area of the wetted portion of the hull. In this relationship, the coefficient of resistance CR=Cfriction+Cwave+Cform. At higher speeds, drag from aerodynamic resistance also contributes to overall drag. Skin friction drag, however, results from water clinging to the hull due to its viscosity and is often the dominant element in the opposing drag force, particularly at high speeds, e.g., above thirty knots. This invention is directed to reducing skin friction drag, which may as well have beneficial impacts on other aspects of ship performance.
As the vessel moves through water, propulsive energy is needlessly expended by pulling an excess mass of water (e.g., momentum transfer) along with the submersed portion of the hull. This results from viscous interaction at a boundary layer between the hull and the surrounding water medium. Varying amounts of water is pulled along with the vessel according to whether the water flow against the hull is laminar or turbulent. Assuming the hull is clean and smooth (which is rarely the case), flow is generally laminar at a leading portion of the bow and, depending on the vessel's speed, becomes turbulent at some point aft of the bow along the bow-to-stern path. Surface texture greatly impacts whether flow is laminar or turbulent. Also, the turbulent flow region of the hull pulls along more water mass than the laminar flow region because turbulent flow reaches deeper into the surrounding bulk of the water medium to disturb more mass. In practice, flow along a hull at normal speeds is mostly turbulent and drag forces resulting from skin friction is directly related to the amount of water mass being pulled along with the vessel's movement through water. According to an aspect of this invention, viscous interaction between a marine hull and surrounding water is reduced by imparting cavitation or microcavitation bubbles in and about the boundary layer at the hull-water interface, which acts to separate the hull from the viscous mass of the water and to reduce momentum transfer to the bulk water medium.
Imparting micron-size air bubbles in the laminar and/or turbulent flow regions of the water immediately against the hull is known to reduce skin friction drag thereby improving the vessel's propulsive performance. In effect, microbubbles may be considered to alter the kinematic viscosity or the effective density and/or viscosity of the water. Drag reduction can be appreciated from the generally accepted hydrodynamic drag force equation FHD=½·CRρυ2A. If the water density ρ decreases due to microcavitation, then so does the hydrodynamic drag force FHD. In addition, such cavitation or microcavitation helps establish an air interface or partial air interface between the hull and the water thereby helping to reduce drag.
According to the present invention, cavitation or microcavitation is accomplished acoustically to effectively fracture the water medium to create, grow, and/or maintain gas pockets in the water medium at and/or within the boundary layer between the hull and the water. The present invention takes advantage of known techniques to acoustically produce and control microbubbles. The invention makes use of entrained air and particulate matter normally extant on the surface of sea, lake, and river water to facilitate nucleation, splitting and growth of microcavitation bubbles through the application of acoustic energy in a controlled fashion.